1. Field of the Invention
The present invention relates to a process for producing an aluminum plate for use as the support of a lithographic printing plate. Specifically, the present invention relates to a method of roughening an aluminum plate for use as the support for a lithographic plate. In particular, the present invention, relates to a method suitable for roughening an aluminum plate which is prone to the appearance of streaks or grainy unevenness in surface quality due to directional differences in crystal grains, both defects being highly likely to occur in the conventional chemical etching techniques.
2. Description of the Related Art
A method commonly adopted for producing aluminum supports for lithographic printing plates is AC etching which employs either an ordinary sinusoidal wave current or a current of a special alternating waveform such as a rectangular wave. By feeding an alternating current with a suitable counter electrode usually made of graphite, the aluminum plate is subjected to roughening, typically in one cycle. The pits produced by this treatment are generally shallow and do not assure a long press life. Under the circumstances, various methods have been proposed with a view to producing aluminum plates that have a grained surface characterized by a uniform and dense distribution of pits having a relatively great depth compared to diameter and which, hence, are suitable for use as the support of printing plates. Examples include roughening with a current from a special electrolytic power supply (Unexamined Published Japanese Patent Application (kokai) No. 67507/1978), adjusting the ratio between the amount of electricity applied in the anode cycle of AC electrolytic roughening and that in the cathode cycle (Unexamined Published Japanese Patent Application (kokai) No. 65607/1979), using a current of a special waveform from a power supply (Unexamined Published Japanese Patent Application (kokai) No. 25381/1981) and a combination of different quantities of current supply per unit area (U.S. Pat. No. 4,272,342 which corresponds to Unexamined Published Japanese Patent Application (kokai) No. 29699/1981).
Conventionally, aluminum supports are produced by the following process: an aluminum ingot is melted and held at a specified temperature to make a slab (400 to 600 mm thick by 1,000 to 2,000 mm wide by 2,000 to 6,000 mm long); an impurity-containing structure of the slab's surface is cut off in thickness of 3 to 10 mm by means of a facing machine; in order to remove the stress within the slab and homogenize its structure, the slab is heat treated by being held in a soaking furnace at 480 to 540.degree. C. for 6 to 12 h; the slab is subsequently annealed to provide a homogeneous rolled structure and finally cold rolled to a specified thickness so that it is straightened to give a high degree of flatness. The thus produced aluminum web has been used as the support of lithographic printing plates.
Aluminum plates can be produced by a process that eliminates the steps of intermediate annealing and soaking treatments from the conventional process of manufacturing aluminum plates. Using such aluminum plates or general-purpose aluminum plates as the support for lithographic printing plates is desirable from the viewpoints of saving energy and effective utilization of resources.
In fact, however, "streaks" and grainy unevenness in surface quality have often appeared when the support lithographic printing plates are manufactured from the above-mentioned aluminum plates. This is usually explained as follows: in the progress of a chemical dissolving reaction in aluminum, the rate of dissolution varies with crystal direction, so the electrochemical pitting reaction of aluminum will proceed at varying rates with the crystal direction.
In short, the asperities produced by the difference in the rate of chemical dissolving reaction are visually seen as streaks or grainy unevenness in surface quality or, alternatively, the difference in pitting reaction depending on the crystal direction (e.g. the difference in the number or size of pits) is seen as streaks or grainy unevenness in surface quality.